Conventionally, in image forming apparatuses such as a multifunction color copying apparatus (an MFP), types of media on which color image and the like are printed increase. Even when printing media are limited to paper, various types of paper having different thicknesses and the like are used.
Such types of paper are usually distinguished by basis weight (unit: g/m2) indicating weight per a fixed are. For example, papers in groups of basis weights 64 to 105, 106 to 163, 164 to 209, 210 to 256, and 257 to 300 are called plain paper, thick paper 1, thick paper 2, thick paper 3, and thick paper 4. These basis weights are usually written on packages of sheets. A user selects one of the groups of the written basis weights, whereby a printing condition corresponding to a type of paper belonging to the selected group is automatically set. In general, it is desirable to change a printing condition according to basis weight. However, basis weight may be unknown. In such a case, it is difficult to easily detect basis weight. It is possible to calculate basis weight from the density and thickness of a sheet. Although it is difficult to measure the density of a sheet, the measurement of paper thickness is relatively easy compared with the measurement of density. Therefore, there is known a device that detects paper thickness in order to learn a paper type and the like.
Such a paper thickness detecting device detects paper thickness in any case. A paper type is estimated from the paper thickness. However, for example, when a device for detecting a paper type such as the paper thickness detecting device is in failure, it may be difficult to detect a paper type. In such a case, there is a problem concerning how a failure of the paper-type detecting device (a media sensor) should be determined. There is also a problem concerning, when a paper type cannot be determined, how a paper type is estimated to perform image formation.
Concerning the determination of a failure of a media sensor, for example, Japanese patent disclosure JP-A-2005-202177 discloses an image forming apparatus that determines a failure of a media sensor on the basis of a media feature value. The image forming apparatus determines that the media sensor is in failure when a feature value S of a medium detected by the media sensor deviates from a range from a detection upper limit value Smax to a detection lower limit value Smin set in advance and performs image formation according to a control mode during a failure set in advance.
Concerning the selection of a paper type in the case of a failure of the media sensor, for example, Japanese patent disclosure JP-A-2005-38277 discloses an image forming apparatus that determines a paper type on the basis of a history of use of sheets in the past. The image forming apparatus stores a history of use of sheets (media), determines, when latest media information cannot be acquired, a type of a medium with a highest frequency of use with reference to the history in the past, and performs processing for print data according to the type.
However, in the determination of a failure of the media sensor, it is unclear in what kind of state a feature value of a medium is measured and it is not easy to determine whether the media sensor is in failure. In the selection of a paper type, a storage device for always storing a history of use of sheets in the past is separately required, which is likely to lead to an increase in cost